1. Field of the Invention
The present disclosure relates to a liquid crystal display device and a method for manufacturing the same, and more particularly, to a liquid crystal display device of a structure for improving black uniformity.
2. Discussion of the Related Art
A liquid crystal display device is widely used in various fields of notebook computers, monitors, spacecraft, aircraft, and etc. owing to advantages of low power consumption based on a low driving voltage and portability.
The liquid crystal display device includes a lower substrate, an upper substrate, and a liquid crystal layer formed between the lower and upper substrates. In the liquid crystal display device, an alignment state of liquid crystal molecules in the liquid crystal layer is controlled based on whether or not an electric field is applied thereto, and light transmittance is controlled according to the alignment state of liquid crystal molecules, whereby images are displayed thereon.
The liquid crystal display device displays a picture image by controlling light transmittance of a liquid crystal using an electric field. This liquid crystal display device is divided into a vertical electric field type liquid crystal display device and a horizontal electric field type liquid crystal display device depending on a direction of an electric field that drives a liquid crystal.
In the vertical electric field type liquid crystal display device, a common electrode formed on an upper substrate and a pixel electrode formed on a lower substrate are arranged to face each other, whereby a liquid crystal of a twisted nematic (TN) mode is driven by the vertical electric field formed between the common electrode and the pixel electrode.
The vertical electric field type liquid crystal display device has an advantage of a high aperture ratio, whereas it has a drawback of a narrow viewing angle at 90 degree.
In the horizontal electric field type liquid crystal display device, a liquid crystal is driven by a horizontal electric field between a pixel electrode and a common electrode in accordance with an in plane switching (IPS) mode, wherein the pixel electrode and the common electrode are formed on one substrate. This horizontal electric field type liquid crystal display device has an advantage of a wide viewing angle at 160 degree. Hereinafter, the horizontal electric field type liquid crystal display device will be described in more detail.
FIG. 1 is a view illustrating a horizontal electric field type liquid crystal display device according to the related art, and FIG. 2A is a cross-sectional view illustrating the liquid crystal display device taken along line A-A′ of FIG. 1.
Referring to FIGS. 1 and 2A, the horizontal electric field type liquid crystal display device according to the related art includes an upper substrate 20, a lower substrate 30, a sealant 42, and an Ag dot 50.
The upper substrate 20 includes a first substrate 21, a black matrix 23, an overcoat layer 25, and a transparent conductive film 22.
The black matrix 23 is formed on one surface of the first substrate 21, the overcoat layer 25 is formed on the black matrix 23, and the transparent conductive film 22, which prevents electro static discharge (ESD) from occurring, is formed on a rear surface of the first substrate 21.
The transparent conductive film 22 is connected with a ground electrode 39 through the Ag dot 50, leaks electro static charges out to the outside, and is formed of indium tin oxide (ITO) which is a transparent conductive material.
Although not shown, the lower substrate 30 includes a second substrate, a thin film transistor, gate lines, data lines, a pixel electrode, a common electrode, and a ground electrode 39.
The upper substrate 20 and the lower substrate 30 are bonded to face each other, and a liquid crystal is injected between the upper substrate 20 and the lower substrate 30.
The sealant 42 is formed along the outside of the upper substrate 20 and the lower substrate 30, and bonds the upper substrate 20 and the lower substrate 30 to each other.
The Ag dot 50 is formed along the outside of the sealant 42, and electrically connects the transparent conductive film 22 of the upper substrate 20 with the ground electrode 39 of the lower substrate 30.
However, the aforementioned horizontal electric field type liquid crystal display device has problems as follows.
FIG. 2B illustrates problems of the liquid crystal display device according to the related art.
As shown in FIG. 2B, one end of the upper substrate 20 may be removed when cutting the upper substrate 20 in a scribing process.
If such removal occurs, one end of the black matrix 23 surrounded by the overcoat layer 25 is exposed, and the exposed portion is shorted from the Ag dot 50. Accordingly, the charges of ESD may flow into the upper substrate 20 along the black matrix 23.
In this way, an irregular voltage may be generated and applied along the black matrix 23, so that a black uniformity defect occurs in a liquid crystal display panel.
The black uniformity defect means that a black region is not formed uniformly on an entire surface of the liquid crystal display panel and a bright spot occurs on some surface of the liquid crystal display panel.